Screw propeller



p 9 1- c. o. KE LL 2,426,617

- SCREW PROPELLER Filed Aug. 25, 193-7 INVNTOR V CLAUDE 0. KELL ATTORNEY Patented Sept. 2, 1947 UNITED STATES PATENT OFFICE 4 Claims.

(Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 O. G. 757) My invention relates to marine screw propellers, and more particularly to a propeller blade the back of which is of such a configuration as to delay or defer the development of back cavitation to increased blade velocities and the face of which, adjacent the leading edge, is shaped to minimize face cavitation.

As is Well understood in connection with marine propellers the face of a blade is the driving face or that surface which propels the water astern when the propeller is in motion; while the back of the blade denotes the surface opposite the face. The different types of cavitation take their names from the sides of the propeller blade where they occur and hence are broadly classified as face cavitation and back cavitation.

Face cavitation generally starts at the root sections which is due in part to the thicker sections at the base of the blades. While this type of cavitation does not have a marked effect upon the efficiency of screw propellers throughout the range of normal operating slips, it is nevertheless objectionable since it is suspected as being the cause of erosion of the blade face and may be the source of objectionable and even serious vibrations. As will be pointed out more in detail hereinafter, face cavitation in accordance with my invention is minimized or substantially eliminated by setting back the blade face adjacent to the leading edge of the blade and at the root sections thereof.

Back cavitation, however, presents a far more serious problem in that when developed to a sufficient degree it reduces propeller thrust and therefore efficiency. Although this type of cavitation cannot be eliminated its occurrence or incidence can be deferred to increased blade velocities. Back cavitation is of two forms: First, that which initially appears as vortices off the blade tips and develops into leading edge cavitation with increase in tip speed and thrust; and second, that which first appears as bubbles forming on the following part of the blade back, which is that part of the back taken with reference to the maximum thickness of the blade sections. This form of cavitation is termed burbling cavitation as distinguished from the first form thereof, namely, leading edge cavitation.

Leading edge cavitation first appears as vortices passing off the blade tips and forming helices in the propeller wake. With increasing speed and thrust there is a corresponding development of these vortices until what appears to be an independent flow is formed across the blade tips, and. bubbles appear down along the leading 2 edge of the blade. As the speed is further increased the width of the flow across the blade tips increases and the trailing edge of this flow passes off into the propeller wake as wider bands of vortices. The cavitation along the leading edge becomes wider and finally when well developed extends across the entire back of the blade as a part of the independent flow which began at the tip sections. This flow remains intact until it is well clear of the following edge of the blade. Soon after leading edge cavitation has developed into an independent flow, as described above, the efficiency of the propeller is found to be affected.

The second type of back cavitation, namely, burbling cavitation, forms on the trailing part of the blade considered with reference to the maximum thickness of the blade sections. At the first appearance of burbling type of back cavitation, separate and distinct bubbles are seen forming on the following half of the blade back. As far as is known at present, these bubbles after forming move in the stream of water flowing across the back of the blade and pass off into the wake of the propeller without collapsing. on the blade back. With decrease of the cavitation index accomplished by increase in speed, with pressure constant, the area affected is found to expand both radially and across the width of the blade towards the maximum thickness of the blade. When completely developed most of the following part of the back of the blade is affected. But even in this stage of development the cavitation is confined to the following half of the blade. With but a slight decrease in the cavitation index after the first bubbles form, the efficiency of the propeller is affected.

From the discussion in the preceding two paragraphs it is clear that leading edge cavitation and burbling cavitation are functions of the cavitation index, increasing with a decreasing cavitation index accomplished by increase in speed, where the cavitation index is defined as the ratio q In this ratio, P0 is the absolute static pressure on any blade section and q is the kinetic energy of the Water relative to the blade section at any given radius. The kinetic energy the relative velocity of flow to the propeller blade sections.

3 With a view to delaying or deferring back cavitation to increased blade velocities I have made an analysis of the factors entering into the phencmenon of water flow around sections at various radii of the propeller blades and have established empirically a relation between the cavita- 7 tion index and the camber ratio,

on the relation between the cavitation index and V camber ratio, is only that part of the phenomenon that develops between the position of maximum thickness of the blade section and the trail-' ing edge. 7

Blades, particularly those of marine propellers, are commonly designed with reference to developed sections of the blade. In forming a developed section, a section of the blade is cut at the desired radius by a cylinder concentric with the axis of the p opeller and the cylinder then developed into a plane parallel with this axis. The

' blade section lying in the plane of the developed cylinder is a developed section and this term is to be understood as designating such a section in the specification and claims.

In the light of the foregoing, it is among the several objects of my invention to provide a blade adapted for movement through a liquid fluid medium in any developed section of which the camber ratio and cavitation index are related in a manner to delay or defer the development of back cavitation; to provide a blade of high efil-.

ciency; and to provide a blade wherein face cavitation is substantially eliminated or minimized.

Other objects and many of the attendant adciatedas the same becomes better understood by reference to the following detailed description when considered in connection With the accom panying sheet of drawings, wherein:

Fig. 1 is a view of a developed section of my.

blade taken at any convenient radius; and

Fig. 2 is a view in elevation of the novel propeller blade of my invention on which the section of Fig. 1 is taken.

In order to formulate the equation Which states empirically the relation that must exist between the cavitation index and the camber ratio if back cavitation is to be deferred, it is necessary that the following factors entering therein be considered:

Let PA =atmospheric pressure.

Pw=pressure on blade section due to depth of water.

P.,=vapor tension of salt water.

m=mass density of sea water.

V=speed of advance, in feet per second, or that speed of the water at the position of the propeller, with reference to some point at an infinite distance. 7

=mcrease of speed of water at the propeller disc due to suction of the propeller, or circulation of water around the blade section. may be taken as a function 01 the'slip, with sufificient accuracy.

vantages of this invention will be readily appre- Vl=speed of water relative to a point on the back of the blade section.

c=the projected blade thickness at the axis of propeller, which is determined by blade thickness ratio or fraction.

cr=maximum thickness of blade section at the radius under consideration.

b=the distance from the point of maximum thickness of the blade section under consideration to the trailing edge of the blade, measured along the chord of the blade section.

g =kinetic energy of water relative to the blade section at the given ra ius.

R=radius of propeller.

r=radius of blade section.

d=diameter of propeller at blade section.

N =revolutions per second.

A=a constant (to be explained below).

From theory, the relative'velocity of Water at a given blade section of a revolving propeller is defined by v. 1rd1v +(v.+%) (1) We further know from hydromechanics that q: /2 mass (velocity) 2 The absolute static pressure on the blade section will be Po=PA+Pw-Pv (3) The maximum thickness of the blade section;

under consideration is obtained by the equation From a consideration of Equation 4 it should Y be clear that the maximum blade thickness of any section is proportional to the distance of the section from the blade tip.

From research work done by me I have determined that if back cavitation is to be delayed or deferred to higher blade velocities, a definite relation must existr between the cavitation index,

Po/q, and the camber ratio, Cr/b. This relation- I ship is defined by the equation 'where A and K are coefficients that are fixed for any one blade. These coeflicients, however,

may vary within certain limits and still permit substantial attainment of the objects of. this invention. The limits of variation are asfollows:

Coefiicient A Coeflicient K 3.750 to 4.167

By suitable computations, as will now be ex I plained, and involving, among other things, the.

aforesaid Equations 4 and 5, the values of Cr, b

and h for each developed blade section are established. The value it is the distance of the position of the greatest blade thickness from the leading edge of the blade section measured along the chord of the section. The computed values" of Cr, b and h for the various sections are tabuf lated and then employed in subsequentgra'phical constructions of the" various developed blade sections.

As a preliminary to any computation of these section values it is assumed that the following i The curve of eiTective horse data are given. power of the ship 'for which the propeller is to be designed, that is, a curve of effective horsepower V plotted against the speed of the ship; the required revolutions of the propeller corresponding to the speeds of the ship; the wake factor or estimated value of the wake as a fraction of the ships speed; the estimated thrust deduction at I the difierent speeds of the ship; andthe'de pth of submergence of the propeller shaft. The proper diameter and pitch of the propeller in question are obtained by several well-known methods 0 to 0.035 I 7 5 of calculatingrthese characteristics... From. a consideration of the thrust: to be. developed 'by'ithe propeller, a blade thickness. fraction or ratio isv assumed from experience which will give suflicient area at the base of the blades to provide ample strength. rigi t With the diameter of the propeller determined from the propulsion characteristics of the ship in question, the maximum blade thickness at the various-blade sections may nowbe computed. By multiplying the blade thickness ratio with the diameter of' the'blade the'value c, which is the projected blade thickness at the axis of the propeller, is determined. The different values of the maximum blade thicknesses, Cr, at the various sections are obtained by use of Equation 4 hereinbefore set forth.

The following procedure is adoptedin computing the various valuesof b at the several blade sections. From average barometric pressures, an assumed temperature of sea water and draft of the ship, the value of P is determined by Equation 3. Knowing the required. speed of revolution, N, for a given ship speed and the speed of advance Va in feet per second, the relative speed Vs at any diameter 'd' can be calculated by employing Equation 1. Having determinedthe relative velocity, Vs, the kinetic energy q is derived by the use of Equation2. By combining the results of Equations 2 and 3, the cavitation index,

may be determined for the various blade sections. Substituting the various values of fl 4 in Equation 5 and solving with the proper values of A and K within the limits hereinbefore specified, the values of the-camber ratio are thereby obtained. Since Or for the" various blade sections has been obtained as previously pointed out, the value of b, the chord length from the position of maximum thickness to the trailing edge of the blade at the radius in question, is determined. It should be clear from the foregoing, that a value of b is determined for each blade section corresponding to the previously calculated values of Cr for thesections in question.

It now is only necessary to determine the remaining section dimension It so that the various developed blade sections may be graphically constructed. It will be remembered that h is the distance of the position of the greatest blade thickness from the leading edge of the blade section measured along chord of the section in question. Based on experimental results I have determined that 71, should bear a definite relation to b if highest efiiciency is to be attained and the development of back cavitation deferred or delayed. The relation that must obtain between I) and h is expressed by the following equation From a tabulation of the. values.cr.,.bvand hior' each blade section, developed blade: sections are now graphically constructed a manner to be ing out the: graphical construction the; length FM equal to or is laid ofi. perpendicular to; the section chord EK, where .EK is. taken equali'inllength'to (b-l-h). It will thus be observed that the maxi- I mum blade thickness c].- has.- the point Moon the section chord situated at a distance: I! from the trailing edge E of the blade and: at a distance: from. the leading ,edge Kthereoiis z-nowilaid off in the manner shown equal to: MEanda circular arc EFE is struck through the points E, F, E with thecenter of the arc lying along. the line. 3 which is an extension of the line FM. Anyof the well known geometrical constructions may be employed in striking the arc EFE. In a similar manner MK is laid off equal to MK and the circulararc KFK struck. withthe center of the arc again .lyi-ng along the" line It. :Circmlar arcs and FK now form-the back'oif thev blade section From a consideration of Equation. .6, it is apparent that b will always be smaller than b soas to insure the attainment .of a. high blade efficiency. From this it necessarily :follows that the radius of curvature of. the circular arc FK will always be less than that of the arc .EF and that the radius of curvature of the arcJFK will be of such a value as to insure highblade eiiiciency. Thus, the developed blade section at any radius will have the back portion thereof between the point of maximum thickness F and the trailing edge E lying substantially along a circular arc; and the camber ratio oithis back portion will be related to the cavitation index in a manner to delay the developmentofback cavitation. Only that part of the phenomenon of back cavitation that develops between the point of maximum thickness F and the trailing edge 'E is controlled by the relation between the cavitation index and the camber ratio as set forth in Equation .5, supra.

As a result of my research work I have determined that face cavitation or cavitation on the face of the blade adjacent the leading edge thereof can be eliminated or greatly reduced by setting back the face of the blade adjacent the leading edge. The characteristics of this correction are dependent on the relative speed Vs and the slip at which the propeller is operating. The face adjacent the leading edge is set back, beginning a short distance back from the leading edge by striking a circular arc MG, the center of whichis likewise situated on the line 3. Generally, this face correction is found necessary only on the root sections of the blade. By setting back the face of the blade adjacent the leading edge in the manner shown in Fig. 1 of the drawings the final blade section EMGF is obtained.

Having determined the shapes of the developed blade sections at the different radii, as outlined above, the strength of the blade is investigated. If the results indicate that any change in sections is necessary, the blade thickness fraction or ratio is corrected accordingly, new section characteristics are computed and the graphical constructions based thereon then carried out all as described hereinbeiore.

According to the provisions of the patent statutes I have set forth the principle and mode of operation of my invention and have illustrated and described. what I now consider to represent its best'embodiment. However, I desire to otherwise than as specifically illustrated and described. V

The invention herein described and claimed may be ,used and/or manufactured by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

I claim:

1. A blade adapted for movement in a liquid fluid medium, said blade having a back which in any developed section of the blade has that portion thereof between its point of maximum thickness and the trailing edge lying substantially along a circular arc, the camber ratio of this back portion being related to the cavitation index according to the equation along a circular arc, the camber ratio of this back portion being related to the cavitation index according to the equation V P,, 0, q- A+ K;-

where A and K are coefiicients that are fixed for any one blade but which may vary respectively from 0 to 0.035 and from 3.750 to 4.167, and the.

distances h and b respectively of the leading and trailing edges measured along'the chord of any section from a point thereon corresponding to the maximum blade thickness being related according to the equation 3. A blade adapted for movement in a liquid fluid medium, said blade having a back which in any developed section of the blade has that p0r-' tion thereof between its point of maximum thickness and the trailing edge lying substantially along a circular arc and the remaining portion thereof between said point of maximum thickness i of the first mentioned back portion being related 1 to the cavitation index according to the equation 0 or. T F where A and K are coeiiicients that are fixed for any one blade but which may vary respectively from 0 to 0.035 and from 3.750 to 4.167.

4. A blade adapted for movement in a liquid fluid medium, said blade having a back which in any developed section of the blade has that portion thereof between its point of maximum thickness and the trailing edge lying substantially along a circular arc, the camber ratio of this back portion being related to the cavitation index according to the equation Where A and K are coefiicients that are fixed for any one blade but which mayvaryrespectively from 0 to 0.035 and from 3.750 to 4.167, and the maximum blade thickness of any section being proportional to the distance of the section from the blade tip.

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